This invention relates specifically to a method and apparatus for curing the coating applied to a can along the seam formed during fabrication of a cylindrical can from a rectangular sheet of can material.
In conventional assembly line methods of manufacturing cans, such as those in which a consumable product is to be packaged, a rectangular sheet of tinplated material is formed into a cylindrical or tubular shape. The ends of the sheet formed into the tubular shape are abutted, crimped, and bonded together as by welding or soldering, so as to form a seam along the entire length of the cylindrical can. The bonding of the sheet ends during the forming of this seam destroys the tin plating and exposes the underlying material. In order to prevent the underlaying material, which may contain toxic substances, from contaminating the product to be packaged, a coating, e.g. vinyl, enamel, is applied to the seam, usually on the inside of the cylindrical can, but often on the outside.
After the coating has been applied to the seam such as by painting or by electrostatic techniques and before further fabrication of the can proceeds, specifically before consumable products are packaged in the can, the coating must be cured, i.e. heated to a specific temperature for a specific period of time. Many other substances, including lacquers and epoxies, may be used as coatings depending upon the particular type of can material in use and the particular product to be packaged in the can. Each of these various types of coatings has its own curing characteristics and must thus be heated to a relatively precise temperature for a specific period of time.
The curing characteristics of some of the coating substances require that the coating be maintained at various levels of temperature for specific periods of time and that it be heated at a precise rate. Improper curing of the coating can result, for example, in boiling off of a portion of the coating if it is a liquid, in overdrying and cracking, or, in the case of low viscosity liquid substances, running of the coating away from the seam.
In one prior art technique, cans which have had their seams coated are placed in a curing furnace, heated by gas or electricity. The furnace raises the temperature of the entire can to a fixed temperature for a period of time sufficient to cure the coating. Because the entire can is heated in the furnace, vasts amounts of energy are wasted. Not only is a significant amount of time required to raise or lower the temperature within the furnace, for the batch processing, but shifting the processing for various can sizes is impossible, and coatings that require precise temperature profiles cannot be accomodated.
The continous process technique of this invention avoids these and other shortcomings attributable to prior art processes and apparatuses used for this purpose.